1. Field of the Invention
The present invention relates generally to an image forming apparatus having resolution varying systems for recording apparatus, and more particularly to an image forming apparatus having a resolution varying system for a printer with which an image high in picture quality can be printed with any selected one of the available resolutions.
2. Related Art
One example of a conventional printer resolution switching system of the image forming apparatus has been disclosed in Unexamined Japanese Patent Application (OPI) No. Hei. 2-206566 (the term "OPI" as used herein means an "unexamined published application").
The system comprises means for switching a plurality of video clock signals different in period according to a resolution switching signal, and means for selecting a signal which is obtained by frequency-dividing a printing start signal by an integer ratio or a divisor ratio of the least common multiple according to the resolution switching signal. With the system, a resolution in the main scanning direction is changed by selecting one of the periods of the plurality of video clock signals, while a resolution in the sub-scanning direction is changed by selecting a signal which is obtained by frequency-dividing the printing start signal by an integer ratio or a divisor ratio of the least common multiple.
With the system, in response to a printing magnification specified by the host apparatus, the resolution can be switched merely by selecting a frequency division signal obtained by frequency-dividing the printing start signal with the speed of rotation of the rotary polygon mirror maintained unchanged. Hence, an operation of switching a resolution to another which is an integer multiple or non-integer multiple of the former can be achieved at high speed.
However, the above-described conventional system suffers from a problem that, when the resolution is lowered, the printed image is much lower in picture quality in the sub-scanning direction.